1. Field of the Invention
This invention relates to a nonwoven fabric in an electret state and a process for its production, as well as a filter material and an air masking material that are made of said nonwoven fabric in an electret state. More particularly, this invention relates to a nonwoven fabric in an electret state that is capable of efficient dust collection and which retains its high collecting performance even under hot and humid conditions. The invention also relates to a process for producing that nonwoven fabric in an electret state, as well as a filter material and an air masking material that are made of said nonwoven fabric.
2. Description of the Related Arts
A polyolefin, for example, a polypropylene is formed into a nonwoven fabric by melt-blow method, which is given a high DC voltage to produce a nonwoven fabric in an electret state. Methods of producing nonwoven fabrics in an electret state by this process are taught in Unexamined Published Japanese Patent Application (kokai) Nos. Sho 60-168511, Hei 2-197110, etc.
Polyolefins modified with at least one monomer selected from among unsaturated carboxylic acids and derivatives thereof can be turned into electrets by processes such as one that is taught in Examined Japanese Patent Publication (kokoku) No. Sho 59-23098. However, the only examples that are described in this publication refer to the measurement of the charge retaining ability of the film electret.
Speaking of the nonwoven fabric in an electret state which is produced by the methods described in Unexamined Published Japanese Patent Application (kokai) Nos. Sho 60-168511, etc., if it is used as a filter, not only is its ability to collect fine dust insufficient in the initial period but retainment of its collecting performance is also poor in a hot and humid atmosphere.